The concept of high-quality cables and related metallic articles was not generally established until the 1970s, but since the development of oxygen-free copper (hereinafter, referred to as “OFC”), the development of cables has rapidly progressed.
Particularly in the case of wire cables making connections in audio systems, etc., it is well known that the quality of cables influences the quality of sound. Thus, replacement of low-quality cables with high-quality cables can provide a great improvement in sound quality.
When such wires are applied with alternating current, an alternating magnetic field will occur, which acts as resistance to the alternating current. Particularly, the higher the frequency, the higher the resistance, and this property is called “inductance”. In the case of cables, if coated wires are coiled in a circular form or formed with positive and negative terminals, the inductance will increase, resulting in a reduction in the purity of sound. However, if coated wires are formed with positive and negative terminals and then twisted like a rope, the inductance will decrease instead, resulting an increase in the purity of sound.
In general, when cables are twisted, the inductance will decrease but the capacitance will increase to make the frequency narrower and the sound muddy. Although it is a clear fact that the sound quality of audio systems differs depending on the cables used, the reason therefor has not yet been clearly established.
For example, an increase in the purity of wires leads to an increase in the purity of sound but generally results in an increase in direct current resistance, which may also reduce the cable's energy. Although the sound quality of cables is influenced by the material and physical properties of the wires themselves, it is also highly influenced by the covering material and crystalline structure of wires, the inductance, capacitance and skin effect caused by terminal processing, and the impedance between audio systems.
Thus, due to the properties of the wires, OFC cables having a purity of 99.99% frequently have better sound quality than 6N copper wire cables having a purity of 99.9999%. Metal conductors for the wires include silver, gold, copper and aluminum, with its electrical conductivity decreasing in descending order thereof. Among them, copper is most frequently used as a conductor, because it is inexpensive and has good electrical conductivity and processability.
Conventional copper wires have a standard purity of 3N (99.9%), and during production, oxygen is blown to increase the workability thereof, while copper produced in an atmosphere having no oxygen is called “OFC”. Other examples of copper wires include 6N (99.9999%), 7N (99.99999%), etc., according to the extent of removal of metals and sulfur, which are impurities other than oxygen. Prior methods for manufacturing copper wires will now be described.
The production of tough pitch copper (hereinafter, referred to as “TPC”) uses a general copper production method, which comprises the steps of introducing oxygen to melt copper, and rapidly cooling the melted copper. This method is suitable for mass production and produces a 3N-purity copper containing sulfur and cuprous oxide and having an oxygen content of 3000-4000 ppm and a purity of 99.9%.
OFC is copper from which the development of high-purity copper started. The OFC is a 4N (99.99%) product which has an oxygen content reduced to less than 10 ppm by removing a cuprous oxide impurity, unlike the existing TPC process utilizing oxygen blown to cool the molten copper. The use of the OFC enables a muddy sound to be removed and the purity of sound to be increased, thus improving the sound clarity.
Linear crystal oxygen-free copper (hereinafter, referred to as “LC-OFC”) was developed based on the theory that every factor reducing the sound quality exists at boundaries between metal crystals. When copper is rapidly cooled in a melted state, it will have a fine crystal structure, in which case an increase in crystals will increasingly interfere with signal transmission. Based on this theory, in order for the OFC to have a unidirectional crystal structure, copper is slowly cooled to obtain large crystals, and the resulting copper crystal structure is then linearly stretched. However, during the process for linearly stretching the copper crystal structure, a mechanical stress and heat are generated in the copper, and the copper crystal structure is adversely affected, thus causing deterioration in sound quality.
Pure crystal Ohno continuous casting (hereinafter, referred to as “PCOCC”) was developed to complement the shortcomings of the LC-OFC, that is, the deterioration in sound quality caused by the production of mechanical stresses and heat in the copper during the process for linearly stretching the copper crystal structure. This continuous casting method produces a single crystal structure by using an additional structure for slow cooling.
Typical examples of conductors having a high strength include LC-OFC and PCOCC, which are not subjected to a thermal treatment process in order to prevent their single crystal structure from being changed. However, as the copper wires are not subjected to the thermal treatment process, they are susceptible to a mechanical stress during the machining process and their crystal structure can be affected. To solve this shortcoming, a myu (m) conductor was developed which is slightly thermally treated to reduce the mechanical stress. In the existing technologies for developing wires, it is recognized that the structure of a single crystal acts as an important factor in manufacturing high-quality cables. However, a suitable technology for preventing the change in the single crystal structure has not yet been developed.
Silver (Ag) has excellent electrical properties, particularly a low electrical resistance, compared to copper. Thus, in audio cables, for example, silver is clearly advantageous compared to copper, particularly when in an oxidized state. In particular, copper is easily oxidized to form a coated film such as a semiconductor film, whereas oxidized silver has an advantage in that it is chemically stable, so that it can sufficiently function as a conductor.
Like the case of copper wires, silver conductors have been continuously improved through an increase in their purity and by the use of thermal treatment. Thus, it is known that recent silver wires and silver-coated OFC wires developed for exclusive use in audio systems carry more information and produce a smoother sound than silver wires for use in general telecommunication purposes.
Aluminum is a conductor having highly specific properties, like silver. It is known that aluminum is slightly higher in electrical resistance than copper but it produces a highly specific sound in a high sound range. Thus, an aluminum wire containing silver or aluminum produces a unique and elegant sound.
Accordingly, to ensure high-quality cables, technologies for manufacturing a copper, silver or aluminum wire using a high-purity single crystal needs to be further developed.